UC Updatable Databases and Applications

Abstract

We define an ideal functionality FUD and a construction ΠUD for an updatable database (UD). UD is a two-party protocol between an updater and a reader. The updater sets the database and updates it at any time throughout the protocol execution. The reader computes zeroknowledge (ZK) proofs of knowledge of database entries. These proofs prove that a value is stored at a certain position in the database, without revealing the position or the value.(Non-)updatable databases are implicitly used as building block in priced oblivious transfer, privacy-preserving billing and other privacypreserving protocols. Typically, in those protocols the updater signs each database entry, and the reader proves knowledge of a signature on a database entry. Updating the database requires a revocation mechanism to revoke signatures on outdated database entries.Our construction ΠUD uses a non-hiding vector commitment (NHVC) scheme. The updater maps the database to a vector and commits to the database. This commitment can be updated efficiently at any time without needing a revocation mechanism. ZK proofs for reading a database entry have communication and amortized computation cost independent of the database size. Therefore, ΠUD is suitable for large databases. We implement ΠUD and our timings show that it is practical.In existing privacy-preserving protocols, a ZK proof of a database entry is intertwined with other tasks, e.g., proving further statements about the value read from the database or the position where it is stored. FUD allows us to improve modularity in protocol design by separating those tasks. We show how to use FUD as building block of a hybrid protocol along with other functionalities.

Concise UC Zero-Knowledge Proofs for Oblivious Updatable Databases

“…In [38], a functionality for a database such that both prover and verifier know the database contents is proposed. The verifier can write information into the database, while the prover performs a read operation similar to the one of F CD .…”

“…Non-hiding VCs are used to construct the functionality. In [39], a variant of the functionality in [38] that interacts with multiple provers and provides unlinkable read operations is defined and constructed by using SVCs. ZK proofs for large datasets.…”